Aqueous pigment inks for ink jet recording that contain a pigment dispersed in an aqueous medium, a resin having an anionic group, and a basic compound have been proposed as ink jet recording inks that can produce recorded images having high water resistance and light fastness on recording materials.
Since pigments themselves are insoluble in ink solvents (such as water and hydrophilic organic solvents), pigments must be finely divided and dispersed. However, coarse particles that cannot be finely divided and remain in an ink solvent or coarse particles that are formed by aggregation as a result of destabilization associated with particle size reduction may cause clogging in an ink flow channel (orifice) of an ink jet printer head and consequently ink ejection failure.
In recent years, the application of an ink jet recording method has rapidly spread not only in offices but also in the field of various traditional printing industries. Thus, high-speed printing and long-term ejection stability has become important. Thus, the demand for ink jet inks that cause no clogging and can be stably ejected has sharply increased.
Methods for removing coarse particles from liquid after dispersion by a physical method, such as filtration or centrifugation, are proposed (Patent Literatures 1 and 2) as means for solving these problems but do not entirely satisfy the recent demands described above.
Coarse particles are preferably reduced before these downstream processes following dispersion rather than by the downstream processes in terms of production efficiency and production yield. The applicants proposed a method for producing an aqueous pigment dispersion for ink jet recording, which involves a kneading process for kneading a mixture of a pigment, a resin, a basic compound, and a wetting agent to produce a colored kneaded product (Patent Literatures 3 and 4). In the kneading process, the pigment is crushed into fine particles, and simultaneously the surface of crushed pigments is coated with the resin. Thus, this method is effective in reducing coarse particles.
However, even these production methods do not entirely satisfy the recent demands for fewer coarse particles.
In order to improve pigment dispersion, a constituent having a long-chain alkyl group is added to a dispersed resin so as to improve the adsorption of a hydrophobic moiety on a pigment (Patent Literatures 5, 6, and 7).